 While this approach worked well during these musical eras, given that the chord progressions were simpler and used less modulation to unusual keys, with the development of bebop in the 1940s, the embellishment and "playing by ear" approach was no longer enough.Although swing was designed for dancing, bebop was not.Bebop used complex chord progressions, unusual altered chords and extended chords, and extensive modulations, including to remote keys that are not closely related to the tonic key (the main key or home key of a song).